The invention relates to a vacuum arc evaporator with which a wide variety of substrates can be provided with various coatings or to which various systems of layers can be applied. Substrates coated in this way can be used in a wide variety of technical fields. The coatings may influence the mechanical properties of components, bring about certain optical effects and also be used, for example, for forming structures in semiconductor technology.
Vacuum arc discharge has been used for some time as a plasma source for the coating of substrates. In this process, minute particles, known as droplets, are produced along with the plasma. These particles are also contained in the finished layer and have an adverse effect on the quality of the layer. Since the layers produced have thicknesses in the nanometer to micrometer range, the relatively small size of these particles is also often troublesome.
New potential applications are, however, imposing ever greater requirements on the quality of the layer, in particular with respect to the electrical, electronic or optical properties. Reference should be made here, inter alia, particularly to the efforts being made in semiconductor technology to achieve miniaturization in the nanometer range.
Since the formation of the particles with rising arc current decreases at an increased rate of movement of the arc spot on the cathode surface, it is endeavored to work in this range. This presents problems, however, since the movement of the arc spot is difficult to control. This has so far been alleviated by operating the arc discharge in a pulsed manner, i.e. extinguishing an ignited arc by lowering the arc current before the bottom point of the arc has moved over the edge of the cathode and then reigniting it in a pulsed manner (cf. DD 280 338).
To at least hinder the embedding of such relatively large-sized particles in such layers and reduce their number, diaphragms serving at the same time as the anode or plasma filters have so far been used, being intended to prevent these particles from reaching the substrate by means of strong magnetic fields.
The magnetic fields used serve the purpose of separating the charged plasma constituents from the neutral particles by using for this purpose a curved path in the direction of the substrate and by the neutral particles not following the curved path.
Since the magnetic fields used influence the arc current, there are design limits to this principle, with the result that losses of coating material due to undesired deposition on the chamber wall, and consequently reduced coating rates, cannot be avoided.
It is therefore the object of the invention to improve vacuum arc evaporators to the extent that an improvement in the quality of the layer can be achieved, with increased material utilization and a higher coating rate.
This object preferably is achieved according to the invention. Advantageous embodiments and developments of the invention will be apparent from the description of the invention provided herein.
The vacuum arc evaporator according to the invention uses the customary elements of a cathode and anode in an evacuable housing. In this case, however, the anode is arranged in such a way that it is surrounded on all sides by the cathode, which consists of an electrically conducting material which is evaporated for forming the coating of a substrate. It goes without saying that the anode is electrically insulated from the cathode, for which purpose it is enclosed by an insulating sleeve, for example of ceramic.
The cathode may be designed in the form of a circular rings at the center of which the anode is arranged. The cathode surface may be planar but also concavely or convexly curved.
The anode may be designed in the form of a rod and protrude with its tip, pointing in the direction of the substrate to be coated, above the surface of the cathode.
The plasma generated as a result of the arc discharge passes to a-substrate, which may be provided with a layer or a system of layers. The layer may be formed from the cathode material, for example a metal or an alloy, or reactively by means of supplied gas as a compound of a metal (nitrides, oxides).
The formation of a system of layers with individual layers of different materials, which may be applied alternately, can also be produced.
For this purpose, it is possible to use a segmented cathode, in which various materials are locally separate from one another. For instance, various alloys may also be deposited in one layer or, for example, reactively carbidic layers may be obtained if one segment of the cathode consists of carbon and another segment consists of a carbide-forming metal.
The vacuum arc evaporator according to the invention may also be operated with a plasma filter, the arrangement of the anode according to the invention having advantageous effects on the concentration of the plasma brought about by the magnetic field used.
The reduced influence of the magnetic field on the plasma and the arc current allows a smaller distance between the magnetic field and the cathode to be maintained.
For igniting the arc discharge there may be at least one igniting device, which may be arranged on the cathode at a distance from the anode. In particular in the case of a segmented cathode, there may be a plurality of igniting devices. In any event, however, a plurality of igniting devices should be arranged symmetrically with respect to one other and with respect to the anode.
A segmented cathode with its various segments may for example be in the form of a figure eight or a clover leaf. The size of the surface of the individual segments does not have to be the same, but instead may vary in dependence on the material-specific rate of movement of the cathode spots or the amount of material to be removed. If, for example, two different materials are to be removed in a certain ratio from various segments for forming a coating, the sum of the pulse durations for the vacuum arc discharge on the respective segments can be chosen in the same ratio.
The various segments may also respectively have an anode assigned to them, so that the arc discharge can be ignited or influenced separately for each segment and the composition of the layer deposited can also be influenced accordingly.
The design according to the invention has advantageous effects in conjunction with an externally applied magnetic field, as a plasma filter. The self-magnetic field of the arc current, the current flow to the anode and the external magnetic field bring about a greater concentration of the plasma, which leads to more effective use of the evaporated material and to an increase in the coating rate. External magnetic fields for guiding the plasma to the substrate may act up to the direct vicinity of the anode or cathode, without disturbing the burning behavior of the arc discharge.
If power circuit breakers with short switching times are used in the arc circuit, short circuits which occur as a result of undesired coatings on the insulation between the anode and the cathode can be easily removed by brief current pulses.
A vacuum arc evaporator according to the invention can be operated with a continuous arc current. In this case, an external magnetic field can guide the vacuum arc with its bottom point in an annular form on the surface of the cathode and prevent the arc from moving toward the anode and remaining there at one point, close to the anode. The ignition of the arc discharge may take place, for example, at the edge of the cathode by means of sliding discharge or contact ignition.
Pulsed operation with a pulsed arc current is also possible, however, as known from DD 280 338, although there the anode is in the form of a diaphragm. By contrast with this, the arc is ignited off-center with a correspondingly arranged igniting electrode, as the igniting device, by a sliding spark discharge between the igniting electrode and the cathode. In the case of a high arc current of I greater than 1 kA, a plurality of cathode spots are formed. The spots form around the anode and run radially outward. The arc current is reduced or switched off entirely before the spots reach the outer edge of the cathode.
Without an igniting electrode, a sliding discharge between the anode and the cathode can likewise lead to the ignition of the vacuum arc.
In the case of a pulsed arc current, the ignition of a vacuum arc can also be initiated by a gas discharge. In this way as well, the ignition should take place off-center, with the aid of a gas supply through the cathode correspondingly arranged there. The pressure stages in the gasp flow may be set such that the gas discharge takes place on the cathode surface between an igniting electrode and the cathode, for which purpose an adequately high negative voltage is applied for a short time.
The ignition may also take place, however, by means of a gas discharge, without an igniting electrode, with a pulsed arc current between the anode and the cathode if an adequately high negative voltage is applied for a short time between the anode and the cathode, the voltage being adequate to ionize the gas.
The ignition may also take place with a pulsed energy beam (for example an electron beam or a laser beam), a plasma being generated locally by the beam focused on the surface of the cathode, for igniting the arc plasma.
It is also possible, however, to work exclusively with a modulated arc current. During the arc discharge, the arc current is kept above 1 kA. After the discharge or when the bottom point of the arc has reached at least the vicinity of the edge of the cathode, the arc current is reduced to a value below 100 A, so that at least one arc spot exists. Under these conditions, the spot runs in the direction of the anode. If the spot reaches the vicinity of the insulation of the anode, a new discharge pulse takes place by increasing the arc current and a plurality of spots are formed, which again run radially to the outer edge of the cathode. After igniting once, this process can be repeated as often as required to form the desired layer. The ignition may take place at the edge of the cathode by means of a sliding discharge or by means of contact ignition with an igniting electrode which can be placed onto the cathode. The arc current modulation may be carried out from a basic current of about 100 A by superposing a current in pulse form. The total amplitude during a pulse may be up to 5000 A.
The invention is to be explained below by way of example.